Deposition on semiconductor wafers is typically achieved by inserting a boat, which holds a number of wafers, into a chamber, commonly known as a tube. While inside the tube, gases are introduced which cause a deposition on the wafers. While the wafers are receiving the deposition, so is the boat which is holding the wafers. The deposition on the boat tends to flake off after removal of the boat from the tube. For example, in the case of depositing phosphosilicate glass (PSG), phosphorous doped silicon oxide, a stainless steel boat is typically used which has a different coefficient of expansion than PSG. Consequently, as a PSG-coated boat changes temperature, PSG particles flake off the boat. Upon removal of the boat from the tube, the wafers can be removed, but not before at least some flaking occurs. If new wafers are placed in the boat, PSG particles from the boat will flake onto those wafers. Subsequent deposition of PSG will not be effected where the particles have flaked onto the wafers. These wafers are then removed and cleaned. The cleaning process removes the particles, leaving a hole in the PSG coating. Any semiconductor die formed in an area which includes such a hole will be defective.
It is desirable to wait until the temperature of a PSG-coated boat has stabilized before placing wafers in the boat. Even so, flaking of PSG particles continues, presumably because they have been loosened during the cool down period. Although quartz boats will reduce flaking substantially, quartz boats are not as durable as stainless steel, and break much more easily. Stainless steel boats for PSG deposition are much more desirable than quartz boats.